Many types of packages are known for electronic components, particularly integrated circuits. Typically, integrated circuits are packaged in structures that consist of the integrated circuit chip bonded to leads, where the chip and the inner portions of the leads are surrounded or encapsulated by a material forming the package body from which the outer ends of the leads protrude. The exposed leads permit the packaged component to be mounted to a printed circuit (PC) board, or in a fixture or socket, or other connecting system.
Conventionally, the material of the package body is made from preformed parts, such as ceramic and occasionally metal, which are then hermetically bonded around the chip, or the material is a plastic and is cast around the chip and the inner portion of the leads in a plastic injection and transfer molding operation. The plastic package is usually not hermetic. Some research is also being conducted in premolding plastic body parts which are then adhesively secured around the integrated circuit in a fashion similar to that of ceramic packages. While ceramic packages are of a higher quality than plastic packages, and more secure from invasion by outside elements such as moisture, they are also more expensive than the plastic packages.
Another issue in packaging electronic components such as integrated circuits, is the material of the leads. Traditionally, lead frames for packages are relatively thick, of about 5 to 10 mils, and the electrical connection from the bonding pads of the integrated circuit to the inner ends of the lead frame are made by very thin wire bonds. In recent years, considerable interest has been generated by using thin foil lead frames or tape, often backed by a layer of polyimide or other plastic. Sometimes the tape has three layers. The advantage of a tape lead frame is that the lead frames can take the form of windows on strips, much like the frames on photographic film, and the bonding process can be automated at high speed to provide a tape automated bonding (TAB) process. The thin tape lead frames typically have a thickness of 1 to 3 mils which permits the package to be made smaller.
Electronic packages using the above described technologies take many forms. One popular form is the dual in-line package or DIP which consists of an elongated package body with leads extending from and turning down on both sides of the package body. Another form is the plastic leaded chip carrier (PLCC) which involves a flat, rectangular or square package body with leads extending from and turning down along all of the four sides of the package body. The ends of the leads may also be configured differently, such as elongated with standoff shoulders for through-hold mounting, or J-lead, gull wing or butt joint for surface mounting.
It is well known that there is constant pressure to provide integrated circuits with higher complexity in smaller packages. This causes the development of DIPs and PLCCs and other packages with very high pin or lead counts where the package itself is very small. Such packages have leads that tend to be fragile and difficult to handle. Often the leads of the package are easily bent. It will be appreciated that TAB technology is employed to solve some of the size requirements, but the thinner leads are even more fragile than leads made using conventional lead frames.
Another requirement of integrated circuit packages is that they must be tested before they are sold to a customer. The handling involved in moving and testing the packages provides additional opportunity for the leads to be damaged. Further, test equipment, particularly test sites, must be devised for each type of package that contains a different number of leads or leads on a different spacing or pitch. Another problem is that a package with leads spaced on a mil system cannot be tested on a tester with metric spaced contacts.
One solution to some of these problems for encapsulated plastic ICs employs a carrier frame. First, a semiconductor die is connected in a die-attach aperture of a copper foil tape. Die contact pads are bonded to the inner ends of interconnected finger contacts on the tape. The finger contacts etched in the foil include splayed out portions extending to probe ends. There are interconnect cross-links that initially connect the finger contacts and the tape edges and function as dam bars in subsequent encapsulation. The die and die bonds are mold encapsulated to form the die package, but also a carrier frame is simultaneously molded around and spaced from the periphery of the die package. The probe ends are exposed within a slot in the frame or extend from the ends of the frame so that probe tips can be pressed thereon to test the die and its bonds. Prior to testing, the interconnects exposed in the annulus between the package and the carrier are blanked out so that each finger leading from a die contact pad becomes discrete, that is, is no longer interconnected to an adjoining finger, so that testing of each die contact and bond can be done. The stiff, molded carrier frame acts to support the probe ends of the fingers and protects and stiffens the foil tape for the testing operations and for shipping and handling purposes. When the package is ready to be mounted, the carrier frame and probe ends are sheared away and discarded and the remaining portions of the fingers are formed into leads to be interconnected to a system, such as on a PC board.
This solution only contemplates that the die package and the carrier frame be molded at the same time using the same encapsulant. Generally, only high quality, thermoset plastics are used in electronic packages. Thus, one problem with this proposed solution is that the volume of the carrier frame is several times, for example, four times, that of the die package, and considerable relatively high quality, expensive plastic is discarded when the carrier frame is sheared off.
An additional problem encountered with prior carrier frame structures is the difficulty in making reliable electrical contact to the probe ends of the leads. In one prior structure, for example, the probe ends are positioned on a shelf or ledge which is an integral part of the carrier frames. The molding of the frame to produce this structure, however, is prone to leaving plastic flash overlying the probe ends. The plastic flash prevents reliable electrical contact to the probe ends during testing. In another prior structure, the probe ends are cut off in close proximity to the carrier frame leaving short "stubs" of the lead ends extending straight out from the frames. The stub ends provide only a limited surface for making contact during test. Additionally, although the lead frame itself may be plated or clad to insure good electrical contact to the ultimate device lead, contact during test is made only to the severed end which is not plated. Acceptable contact to the severed end can be made when the end is freshly cut, but reliability of the contact deteriorates with aging, especially with the time and temperature that may be encountered during life or accelerated testing.